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| United States Patent |
5,254,530
|
|
Sugimoto
, et al.
|
October 19, 1993
|
MOCVD of a-axis or b-axis oriented superconducting Bi-Sr-Ca-Cu-O films
Abstract
A Bi-Sr-Ca-Cu-O-type superconductive film is formed on an MgO (100) single
crystal substrate by the chemical vapor deposition method at a film
formation speed of 780.degree. C. or less and a film formation speed of
1.0 nm/min or more, and exhibits an a-axis or b-axis preferential growth
with respect to the substrate surface.
| Inventors:
|
Sugimoto; Tsunemi (Yamaguchi, JP);
Sugawara; Kazushi (Tokyo, JP);
Nakagawa; Mikio (Tokyo, JP);
Shiohara; Yuh (Tokyo, JP)
|
| Assignee:
|
International Superconductivity Technology Center (Tokyo, JP);
Ube Industries, Ltd. (Yamaguchi, JP);
Sharp Corporatoin (Osaka, JP);
Fujikura Ltd. (Tokyo, JP)
|
| Appl. No.:
|
900370 |
| Filed:
|
June 18, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
505/447; 427/62; 427/126.3; 427/255.32; 427/314; 428/930; 505/730; 505/734 |
| Intern'l Class: |
C23C 016/00 |
| Field of Search: |
505/1,730,734
427/62,63,255.3,255.2,255.1,126.3,314
428/930
|
References Cited
| Foreign Patent Documents |
| 446145 | Sep., 1991 | EP.
| |
| 1-257194 | Oct., 1989 | JP.
| |
| 2-252618 | Nov., 1990 | JP.
| |
| 9107236 | May., 1991 | WO.
| |
| 9119026 | Dec., 1991 | WO.
| |
Other References
Sugimoto et al., "Metalorganic Chemical vapor deposition of B.sub.i
--S.sub.r --C.sub.a --C.sub.u --O ultrathin films", J. Appl. Phys. 70(3)
Aug. 1991, pp. 1600-1605.
Endo et al., "Growth and properties of B.sub.i -S.sub.r -C.sub.a -C.sub.u
-O superconducting films by MOCVD", Thin Solid Films, 206, Dec. 1991 pp.
125-127.
Natori et al., "Superconducting B.sub.i -S.sub.r -C.sub.a -C.sub.u -O Thin
Films Grown by Metalorganic Chemical Vapor deposition at different
temperatures", Jpn. J. Appl. Phys. 28(9) Sep. 1989 pp. 1578-1580.
Yamane et al., "Preparation of B.sub.i -S.sub.r -C.sub.a -C.sub.u -O films
by chemical vapor deposition with metal chelate and alkoxide"Chemistry
letters (1988) pp. 1515-1516.
|
Primary Examiner: King; Roy
Attorney, Agent or Firm: Kanesaka and Takeuchi
Claims
What is claimed is:
1. A method of fabricating a Bi-Sr-Ca-Cu-O superconductive film comprising,
forming a Bi-Sr-Ca-Cu-O superconductive film on an MgO (100) single crystal
substrate by a chemical vapor deposition method at a substrate temperature
of 780.degree. C. or less and a film formation speed of 1.0 nm/min to 2.5
nm/min so that the Bi-Sr-Ca-Cu-O superconductive film consisting
essentially of an a-axis or b-axis orientation with respect to a substrate
surface is grown on the substrate, said chemical vapor deposition method
including reaction gases of organometallic complexes of Bi, Sr, Ca and Cu,
a carrier gas and an oxidizing gas, partial pressure of said oxidizing gas
being from 1 to 10 torr.
2. A method according to claim 1, wherein said Bi-Sr-Ca-Cu-O
superconductive film contains Pb.
3. A method according to claim 1, wherein ligand of said organometallic
complexes is at least one selected from the group consisting of
acetylacetone, dipivaloylmethane, cyclopentadiene, and a compound as can
be represented by the following formula:
R--CO--CH.sub.2 --CO--C(CH.sub.3).sub.3
where R indicates a fluorinated lower alkyl group of a carbon number of
1.about.4.
4. A method according to claim 3, wherein said fluorinated lower alkyl
group represented by R in the above formula is selected from the group
consisting of trifluoromethyl group, pentafluoroethyl group, and
heptafluoropropyl group.
5. A method according to claim 1, wherein ligand of said complexes is at
least one selected from the group consisting of phenyl group, methyl
group, ethyl group and aryl group.
6. A method according to claim 1, wherein the organometallic complex of Bi
is at least one selected from the group consisting of Bi(ph).sub.3,
Bi(DPM).sub.3, BiMe.sub.3, BiEt.sub.3 and Bi alcoholate,
the organometallic complex of Sr is at least one selected from the group
consisting of Sr(DPM).sub.2 and Sr(PPM).sub.2,
the organometallic complex of Ca is at least one selected from the group
consisting of Ca(DPM).sub.2 and Ca(PPM).sub.2,
the organometallic complex of Cu is at least one selected from the group
consisting of Cu(DPM).sub.2, Cu(PPM).sub.2 and Cu(acac).sub.2.
7. A method according to claim 1, wherein said carrier gas is at least one
selected from the group consisting of Ar, He, Ne and N.sub.2.
8. A method according to claim 1, wherein said oxidizing gas is at least
one selected from the group consisting of O.sub.2, O.sub.3, air, N.sub.2 O
and NO.sub.2.
9. A method according to claim 1, wherein the substrate temperature is
780.degree. to 750.degree. C. and the film formation speed is 1.0 to 2.0
nm/min.
10. A method according to claim 1, wherein the thickness of said
Bi-Sr-Ca-Cu-O-type superconductive film is 100 to 1000 nm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a Bi-Sr-Ca-Cu-O
superconductive film and, in particular, to a superconductive film of an
a-axis or b-axis preferential orientation with respect to a substrate
surface, which is very effective for Josephson devices, three-terminal
devices or the like.
2. Description of the Related Art
Because of the high superconductivity critical temperature (Tc), oxide-type
superconductive films are expected to be applied to electronic devices,
such as transistors and Josephson junction devices. However, the
superconductivity of an oxide-type superconductive film exhibits a strong
anisotropy, so that it requires a film orientation control. In the case of
device application, in particular, a device structure whose ab crystal
surface with a large coherence length (.zeta.) allows a flow of electric
current is advantageous from the viewpoint of the properties of the device
to be obtained. For this purpose, it is important that the film should be
of a-axis or b-axis orientation with respect to the substrate surface.
Conventionally known methods of fabricating oxide-type superconductive
films include the PVD (physical vapor deposition) method and the CVD
(chemical vapor deposition) method. The lower limit of the film formation
speed in the PVD method is relatively high as compared to that in the CVD
method, and a reduction in speed in the PVD method will result in the
irregular film formation speed and the film composition, thereby making it
difficult to obtain an ultra-thin film which is considered producible at
low film formation speed (Applied Physics Letters. Vol., 53, No. 7, 1988,
pp. 624).
Generally speaking, it is possible with the CVD method to maintain the
saturated vapor of the material compound stable within a fixed temperature
range. Further, before they are caused to react so as to effect film
formation on a substrate, the vapor and the oxidizing gas are separated
from each other, so that it is possible to cause the requisite reaction to
take place exclusively on the substrate (Journal of Applied Physics, Vol.
67, No. 3, 1990, pp. 1562). In this respect, the method is superior to the
PVD method, in which it is difficult to control the reaction between the
material gas and the oxidizing gas and the material vapor concentration in
the system is unstable.
Thus, it is more advantageous from the industrial point of view, to
fabricate oxide-type superconductive films by the CVD method.
As stated above, when using oxide-type superconductive films for various
types of devices, an a-axis or b-axis film orientation is desirable.
However, because of their strong anisotropy in crystal structure,
oxide-type superconductive films have been inclined to c-axis orientation,
which is particularly true for the Bi-Sr-Ca-Cu-O-type superconductive
films.
There have conventionally been reported examples of a Bi-Sr-Ca-Cu-O-type
superconductive film in which an a-axis or b-axis preferential growth is
effected by sputtering, as well as examples of a Y-Ba-Cu-O-type
superconductive film in which an a-axis or b-axis preferential growth is
effected by the CVD method. However, there has been provided as yet no
Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation
prepared by the CvD method.
Thus, there has been a request for a formation of a Bi-Sr-Ca-Cu-O-type
superconductive film of a-axis or b-axis orientation with respect to the
substrate surface by the CVD method.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a A Bi-Sr-Ca-Cu-O-type
superconductive film fabrication method which makes it possible for a
Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis
preferential growth with respect to the substrate surface, which is useful
as a material for various types of devices, to be provided by the CVD
method, which is advantageous from the industrial point of view in forming
oxide-type superconductive films.
In accordance with a first aspect of the present invention, there is
provided a Bi-Sr-Ca-Cu-O-type superconductive film formed on an MgO (100)
single crystal substrate by the chemical vapor deposition method and
featuring an a-axis or b-axis preferential growth with respect to the
substrate surface.
In accordance with another aspect of the present invention, there is
provided a method of fabricating a Bi-Sr-Ca-Cu-O-type superconductive film
according to the first aspect of the present invention, the method
featuring a film formation on an MgO (100) single crystal substrate by the
chemical vapor deposition method, at a film formation temperature or
substrate temperature of 780.degree. C. or less and a film forming speed
of 1.0 nm/min or more.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described in detail.
Examples of the chemical composition of an Bi-Sr-Ca-Cu-O-type
superconductor forming a Bi-Sr-Ca-Cu-O-type superconductive film according
to the present invention include the following:
Bi.sub.2 Sr.sub.2 Ca.sub.Cu.sub.2 O.sub.x
Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y
To improve Tc (the critical temperature), the above compositions may
further include Pb.
A Bi-Sr-Ca-Cu-O-type superconductive film according to the present
invention as described above is formed on an MgO single crystal substrate
of a plane index of [100] by the CVD (chemical vapor deposition) method.
In the following, a method of fabricating a Bi-Sr-Ca-Cu-O-type
superconductive film according to the present invention will be described.
A Bi-Sr-Ca-Cu-O-type superconductive film according to the present
invention is fabricated by the CVD method, according to which a thin film
is formed through deposition on an MgO (100) single crystal substrate,
using reactant gases, a carrier gas, and an oxidizing gas in conformity
with fixed chemical compositions.
Examples of the material gases used in the present invention include
organometallic complexes of Bi, Sr, Ca, Cu, or the like.
Examples of the organic portions of the organometallic complexes, i.e., the
complex ligands, include: acetylacetone (hereinafter abbreviated as
"acac"), dipivaloylmethane (hereinafter abbreviated as "DPM"),
cyclopentadiene, and further, a compound as can be represented by the
following formula:
R--CO--CH.sub.2 --CO--C(CH.sub.3).sub.3
(where R indicates a fluorinated lower alkyl group of a carbon number of
1.about.4.)
Use of such ligands facilitates the synthesis and isolation of the metal
complexes and enables the vapor pressure of the organometallic complexes
to be low, so that it is very advantageous in preparing the reactant
gases. Other examples of the ligands include phenyl group (hereinafter
abbreviated as "ph"), alkyl group and aryl group. The alkyl group
comprises methyl group (hereinafter abbreviated as "Me"), and ethyl group
(hereinafter abbreviated as "Et"). Examples of the fluorinated lower alkyl
group represented by R in the above formula include trifluoromethyl group,
pentafluoroethyl group (hereinafter abbreviated as "PPM"), and
heptafluoropropyl group.
Examples of the organometallic complexes used as the material gases include
the following:
Bi(ph).sub.3, Bi(DPM).sub.3,
BiMe.sub.3, BiEt.sub.3, Bi alcoholate,
Sr(DPM).sub.2, Sr(PPM).sub.2,
Ca(DPM).sub.2, Ca(PPM).sub.2,
Cu(DPM).sub.2, Cu(PPM).sub.2, and Cu(acac).sub.2
Examples of the carrier gas used in the present invention when feeding the
reactant gases to the reactor include inactive gases, such as Ar, He and
Ne, and further, N.sub.2.
Examples of the oxidizing gas used in the present invention include
O.sub.2, O.sub.3, air, N.sub.2 O and NO.sub.2. The partial pressure of the
oxidizing gas in the entire gas is preferably 0.01 to 760 torr and, in
particular, 1 to 10 torr.
In film formation, the temperature of the MgO (100) single crystal
substrate, i.e., the film formation temperature, is set at 780.degree. C.
or less. A film formation temperature higher than 780.degree. C. would
make it impossible to obtain a Bi-Sr-Ca-Cu-O-type superconductive film of
an a-axis or b-axis preferential orientation with respect to the substrate
surface.
The film formation speed is set at 1.0 nm/min or more. A film formation
speed less than 1.0 nm/min would make it impossible to obtain a
Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis
preferential orientation with respect to the substrate surface.
In the present invention, it is particularly desirable that the film
formation be conducted at a film formation temperature or temperature
substrate of 780.degree. to 750.degree. C. and a film formation speed of
1.0 to 2.0 nm/min.
Usually, a preferable thickness of a Bi-Sr-Ca-Cu-O-type superconductive
film obtained in this way is in the range of approximately 100 to 1000 nm.
The Bi Sr-Ca-Cu-O-type superconductive film of the present invention is a
high-property superconductor in which Tc is generally 50 to 100K, and
exhibits an a-axis or b-axis preferential orientation in growth with
respect to the substrate surface, so that it is very useful for various
types of devices.
By using an MgO (100) single crystal as the substrate and effecting film
formation at a film formation temperature of 780.degree. C. or less and a
film formation speed of 1.0 nm/min or more, it is possible for a
Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation
with respect to the substrate surface to be easily obtained by the CVD
method.
EXAMPLES
The present invention will now be described more specifically with
reference to examples and comparative examples. In the following, gas flow
rate will be given in sccm, in terms of cc/min at 1 atm and 25.degree. C.
EXAMPLE 1
By using reactant gases shown in Table 1, a Bi-Sr-Ca-Cu-O-type
superconductive film was fabricated. The substrate used and the
fabricating conditions are shown in Table 2.
TABLE 1
______________________________________
Reactant gas
Temperature and flow rate*
______________________________________
Bi(ph).sub.3
87.degree. C., 50 sccm
Sr(DPM).sub.2
215.degree. C., 50 sccm
Ca(DPM).sub.2
183.degree. C., 50 sccm
Cu(DPM).sub.2
102.degree. C., 50 sccm
______________________________________
*Temperature and flow rate when the evaporated material gases are caused
to flow out into the reactor along with Ar as the carrier gas.
TABLE 2
______________________________________
Oxidizing gas O.sub.2
______________________________________
Substrate MgO single crystal (plane
index: [100])
Substrate temperature
780.degree. C.
Total pressure 10 torr
(Reactant gases + Ar + O.sub.2)
O.sub.2 partial pressure
approximately 6.7 torr
O.sub.2 flow rate 400 sccm
Film formation speed
approximately 1 nm/min
______________________________________
As a result, a film having a thickness of approximately 130 nm was formed
after a film formation process of approximately 2 hours. The properties of
the Bi-Sr Ca-Cu-O type superconductive film thus obtained were as follows:
XRD spectrum: a crystal substrate of a-axis or b-axis orientation
exhibiting peaks corresponding to the plane indexes of (100), (200),
(300), and (400).
Chemical composition: Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x
Critical temperature: approximately 50 to 60K
EXAMPLES 2.about.6, COMPARATIVE EXAMPLES 1.about.9
Film formation was conducted in the same manner as in Example 1, except
that the film formation speeds and film formation temperatures (substrate
temperatures) shown in Table 3 were adopted. The crystal orientation of
each of the films obtained was measured by XRD and evaluated by the
following criteria. The results are shown in Table 3 together with Example
1.
Criteria for Evaluation:
.largecircle.=A Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or
b-axis preferential orientation was obtained.
.DELTA.=A Bi-Sr-Ca-Cu-O-type superconductive film including a component of
a-axis or b-axis orientation and a component of c-axis orientation in the
same proportion was obtained.
X=A Bi-Sr-Ca-Cu-O-type superconductive film of a c-axis preferential
orientation was obtained.
TABLE 3
______________________________________
Film
formation
temperature
Film formation speed (mm/min.)
(.degree.C.)
2.5 1.0 0.5
______________________________________
720 .largecircle. Example 2
.largecircle. Example 3
.DELTA. Comp. Ex. 1
750 .largecircle. Example 4
.largecircle. Example 5
.DELTA. Comp. Ex. 2
780 .largecircle. Example 6
.largecircle. Example 1
.DELTA. Comp. Ex. 3
820 .DELTA. Comp. Ex. 4
X Comp. Ex. 5
X Comp. Ex. 6
850 X Comp. Ex. 7
X Comp. Ex. 8
X Comp. Ex. 9
______________________________________
It is obvious from Table 3 that in accordance with the present invention, a
high-property Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or
b-axis preferential orientation can be obtained.
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